In the semiconductor manufacturing industry, it is necessary to achieve precise control of the quantity, temperature and pressure of one or more reactant materials which are delivered in a gaseous state to a reaction chamber. Mass flow controllers are widely used in the semiconductor manufacturing industry to control the delivery of process reactants. A mass flow controller generally includes a mass flow rate sensor for measuring the rate of flow of gas through the controller, a valve for controlling the flow of gas through the controller and a computer connected to the mass flow rate sensor and the valve. The computer is programmed with a desired flow rate, which the computer compares to an actual flow rate as measured by the mass flow rate sensor. If the actual flow rate does not equal the desired flow rate, the computer is further programmed to open or close the valve until the actual flow rate equals the desired flow rate.
Thermal mass flow controllers operate on the principle that the rate of heat transfer from the walls of a flow channel to a fluid flowing in laminar flow within the channel is a function of the difference in temperatures of the fluid and the channel walls, the specific heat of the fluid, and the mass flow rate of the fluid. Thus, the rate of mass flow of a fluid (in the laminar flow regime) can be determined if the properties of the fluid and the temperatures of the fluid and tube are known.
One class of thermal mass flow rate sensors employs a capillary tube as the primary sensing mechanism, as shown in the exemplary prior art mass flow rate sensor 10 of FIG. 1. In such a device, a capillary tube 12 diverts a portion 14 of the main flow 16 propagating through the primary conduit 18. It is important to note that this figure is not necessarily to scale. Typically the capillary tube 12 is significantly smaller than the primary conduit 18, but is shown somewhat large in FIG. 1 for clarity. Generally one or more heating elements 20 attach to the capillary tube 12 to allow a heat transfer from the heating elements 20, through the tube 12 and to the fluid. The heating elements 20 also serve as resistance temperature sensors that track the local temperature of the tube wall. Heat transfer between the fluid 14 flowing in the capillary tube 12 from the tube walls is a function of the difference between the fluid temperature and the wall temperature, and the heat transfer rate coefficient inside of the tube. The increase in gas temperature between the two coils is a function of the mass flow rate of the gas and the specific heat of the fluid. A circuit converts the difference in resistance (or temperature) of the two coils 20 into a voltage output which is calibrated to a known flow source.
Ideally, the output of the sensor of FIG. 1 should be a linear function of the flow rate Q through the sensor. For practical sensors, however, the output of the sensor is a linear function of the flow rate Q through the sensor only for a limited range. At an upper limit flow rate, the practical curve diverges from the ideal curve, i.e., becomes nonlinear, Practical sensors consequently operate with either a reduction of flow range of an increase in error due to the nonlinear signal. In addition, the degree of non-linearity of the output of the practical sensor at higher flow rates is highly dependent on gas properties, which limits the accuracy of the gas correction factors used for to convert the calibration to other gasses.
It is an object of the present disclosure to substantially overcome the above-identified disadvantages and drawbacks of the prior art and provide an improved thermal mass flow rate sensor having increased sensitivity, range and accuracy. Preferably, the improved thermal mass flow rate sensor will provide an increased heat transfer rate between the heating elements and fluid in the capillary tube of the sensor.